readObject

Read an instance from the stream. The instance will have been written by the
Externalizer.writeObject(ObjectOutput, Object) method. Implementations are free
to create instances of the object read from the stream in any way that they
feel like. This could be via constructor, factory or reflection.

getId

Returns an integer that identifies the externalizer type. This is used
at read time to figure out which AdvancedExternalizer should read
the contents of the incoming buffer.
Using a positive integer allows for very efficient variable length
encoding of numbers, and it's much more efficient than shipping
AdvancedExternalizer implementation class information around.
Negative values are not allowed.
Implementers of this interface can use any positive integer as long as
it does not clash with any other identifier in the system. You can find
information on the pre-assigned identifier ranges in
here.
It's highly recommended that maintaining of these identifiers is done
in a centralized way and you can do so by making annotations reference
a set of statically defined identifiers in a separate class or
interface. Such class/interface gives a global view of the identifiers
in use and so can make it easier to assign new ids.
Implementors can optionally avoid giving a meaningful implementation to
this method (i.e. return null) and instead rely on XML or programmatic
configuration to provide the AdvancedExternalizer id. If no id can be
determined via the implementation or XML/programmatic configuration, an
error will be reported. If an id has been defined both via the
implementation and XML/programmatic configuration, the value defined via
XML/programmatic configuration will be used ignoring the other.

getTypeClasses

Returns a collection of Class instances representing the types that this
AdvancedExternalizer can marshall. Clearly, empty sets are not allowed.
The externalizer framework currently requires all individual types to be
listed since it does not make assumptions based on super classes or
interfaces.